Method of enhancing the cold radial expansion properties of hollow brass rivets



United States Patent Ofiice 3,022,525 Patented Feb. 27, 1962 METHOD OF ENHANCING THE COLD RADIAL EXPANSION PROPERTIES OF HOLLOW BRASS RIVETS Alan R. Pels, Stamford, Conn., and Walter A. Backofen, Marblehead, Mass assignors to National Distillers 8: Chemical Corporation, Bridgeport, Conn., a corporation of Virginia No Drawing. Filed Dec. 30, 1958, Ser. No. 783,722

1 Claim. (CI. 10-27) This invention is concerned with improving the cold radial expansion properties of brass wire.

Rivets of the hollow or tubular shank type are made from tubular rivet stock wire drawn from brasses which may range from a substantially 100% copper with either only a trace or possibly no zinc content up to a zinc content of around 35%, the balance being substantially all copper. These percentages are by weight in all instances. Such alpha brasses are cold drawn extensively when making the wire of the desired diameters and in doing this the elongation apparently imparts to the metal a longitudinally extending fibrous or striated structure. This structure persists even after annealing.

Hollow shanked rivets made from such material exhibit a radial splitting characteristic when upset to effect their fastening function. In other words, when such a hollow shank or stud is expanded by force directed axially against it the resulting radial flange produced is apparently weakened by radially extending fractures presumably representing lines of weakness induced by the extensive elongation required to bring the material to the diameter desired from the much thicker starting material. This radial splitting effect is undesirable.

With the foregoing in mind one of the objects of the present invention is to eliminate the described undesirable radial splitting or fracturing or rupturing tendency from wire intended for rivets and the like and made of one of the alpha brasses or, in other Words, from metals of the class consisting of copper and copper and zinc alloys up to the approximate 35% zinc limit marking the limit of the alpha brasses, generally speaking.

Other objects may be inferred from the following.

According to the present invention the described brass wire may be produced by conventional methods down to the point where it has been cold drawn almost to gage. This involves the usual extensive elongation required for the majority of the thickness reduction and incidentally introducing the radial splitting problem discussed hereinabove.

This brass wire thus reduced is twisted about its axis by the application of adequate torque, at least enough to permanently deform the wire when thus twisted. That is to say, the wire should be twisted about its axis at least to an extent preventing it from springing back. The wire should be plastically deformed by the twisting.

As the wire is twisted its thickness is reduced and that is why it is not drawn completely to gage when this invention is used. The wire must be drawn to a thickness permitting this twisting with its incidental gage reduction without this reducing the wires diameter so far as to exceed the dimensional tolerances of the wire gage being produced. The wire should not be twisted so far as to physically damage the wire. During the twisting enough tension should be used to keep the wire straight and avoid kinking of the wire but the tension should not be enough to permanently stretch the wire in pure tension without the twisting. V

With the above treatment the processing of the wire may continue in the usual manner. Preferably the annealing required after the extensive wire drawing effecting the majority of the reduction required, is performed prior to the twisting since it greatly facilitates the permanent setting of the wire in a permanently twisted form about its axis. Being then only slightly cold work hardened or tempered it may be desirable, and it is perfectly feasible, to give the wire one or two cold drafts through a die to raise its temper further as required for rivet stock. If this subsequent cold drawing is practiced the diameter to which the wire is drawn by conventional methods, prior to the twisting, must make the necessary allowance required for production of the gage desired. The wire should not be heavily reduced with consequent extensive elongation, following the twisting step.

Brass wire processed as described exhibits a markedly reduced tendency to crack, rupture or fracture radially when in the form of a hollow rivet shank or the like which is upset radially in the normal manner. Fine or hairline cracks may develop but usually the more extensive type of failure, which apparently reduces the strength of the radially expanded metal, will not result.

It is believed that any extensively reduced metal which when elongated greatly possesses a fibrous characteristic, might also be benefited by the described treatment. When the fibrous effect is parallel to the axis of the wire and the latter is radially deformed the weakening effect of the fibers is exerted substantially at right angles to the radial expansion of the material so that the described cracking would seem to be practically inevitable. By giving the material a twist about its axis with the twist permanently set, the fibers are oriented at an angle with respect to the expanding force so that the objectionable cracking action would seem to be reduced in such instances also.

However the trouble has been particularly observed in connection with the described type of brasses. Microscopic examinationof such brasses does not necessarily lead to the conviction that they are fibrous in character. Possibly such brasses are not in fact fibrous but even so the use of the permanently set twist technique described hereinabove does in fact appreciably and advantageously reduce the radial cracking trouble when such wire is in the form of a tubular rivet shank or the like which is radially upset.

Summarizing the foregoing it can be seen that in effect the present invention provides a new process for making hollow rivets or other tubular parts particularly when they are made from brass wire containing not more than about 35% zinc. This process comprises the step of reducing wire stock made of such brass to the desired finished thickness, less the diameter required for the subsequent working, this operation involving -a substantial amount of wire drawing work while the brass is cold.

' This necessary reduction step not only hardens the wire but introduces the trouble of elongating the structural components of the wire so as to introduce the fibrous or striated effect. This effect extends lengthwise with respect to the wire substantially parallel to the axis of the latter. Then follows the annealing and the twisting of the wire about its axis with suflicient rotation to give it a permanently twisted set, followed usually by the temper cold drawing, after which the wire may be formed into the rivets so that the wire forms the hollow rivet shanks or the like which are radially upset eventually.

The wire is normally cold worked to temper it after the twisting and before the formation of the rivets, as described, but this cold working is limited to a degree avoiding materially reorienting the wires structural components from the twist or helical structure'imparted by the twisting.

Any degree of twist that is permanent should be bene- 'ficial but a 45 minimum permanent twist set produces appreciable results. This is measured by the circumferential displacement of the surface of the wire relative V g 3 r to its neutral axis which does not twist, at every transverse plane through the length of the wire. Obviously the actual number of twists given a length of wire depend on the extent of thislength.

We claimzr r J I V 1 A process for making hollow'rivets from brass wire containing not more than about 35% zinc, said process comprising elongating wire made of said brass, annealing said wire, twisting said wire about its axis with sufficient rotation to give it a permanently twisted set, and forming said rivets from said wire so that the latter forms the hollow rivet shanks for radial upsetting thereof, said 262,850 Stiff Aug. 15, 1882 2,250,610 Simons .Q July 29, 1941 2,361,771 Huck Oct. 31, 1944 

